Check that the code is typed exactly as the sending Mac shows it, and that the sending Mac has started sending. Both need an internet connection to find each other by code.

The password on the receiver doesn't match the sending Mac's, or one end is running an older version. Type the password again on both ends, and update both to the current version.

• On the sending Mac, the Video source must be a picture source, not "Receiver uses local file".
• Wait a few seconds: the picture starts at the next full frame.
• If it still doesn't appear, disconnect and connect again.

• Raise the buffer (Up on the Apple TV remote while watching). 500 ms is a good setting on Wi-Fi. The sound waits by the same amount, so they stay in sync.
• Lower the video bitrate on the sending Mac (3–4 Mbps works well over Wi-Fi and the internet).
• Use Ethernet for the Apple TV if you can.
• Turn on Settings › Video and Audio › Match Content › Match Frame Rate on the Apple TV for the smoothest motion.

TVs delay the picture by different amounts. On the Apple TV's first screen, use Picture timing to show the picture earlier or later, in 20 ms steps, until it lines up.

Under Speakers, choose the layout the sending Mac's channels are in: Stereo, or 5.1 / 7.1 in SMPTE or Pro Tools (film) order. Transmitter TV routes each channel to the right speaker whatever order your TV or receiver uses.

Make sure both are on the same network (not a guest network). Use the address the Apple TV shows on its first screen. With a mesh Wi-Fi system, connecting by code also works within one building.